In the drilling of oil and gas wells, CWD is a method of forming a wellbore with a drill bit attached to the same string of casing that will line the wellbore. In other words, rather than run a drill bit on smaller diameter drill string, the bit is run at the end of larger diameter casing that will remain in the wellbore and be cemented therein. Because the same string of casing transports the bit and lines the wellbore, no separate trip out of or into the wellbore is necessary between the forming of the borehole and the lining of the borehole. CWD is especially useful in certain situations where an operator wants to drill and line a is wellbore as quickly as possible in order to minimize the time the borehole remains unlined and subject to collapse or the detrimental effects of pressure anomalies.
After drilling to a predetermined depth, the drill bit is destroyed or retrieved from the borehole and, thereafter, a cementing operation is performed. The cementing operation fills the annular space between the outer diameter of a casing and the borehole wall with cement. The cement will set the casing in the wellbore and facilitate the isolation of production zones and fluids at different depths within the wellbore.
A number of conventional methods exist by which to retrieve the bottom hole assembly. For example, one method involves destroying the drill bit using an explosive charge and thereafter removing the bottom hole assembly. Another involves deploying a retrieval tool down the wellbore that latches onto the bottom hole assembly and thereby removes it. However, the disadvantage of such methods is that they are dangerous, complicated and time-consuming.
Accordingly, there is a need in the art for a more safe, practical and efficient technique in which to retrieval a bottom hole assembly during a CWD operation.